Symbiodinium clade C generality among common scleractinian corals in subtropical Hong Kong

The identity of coral-hosted dinoflagellates (genus Symbiodinium) is a critical factor that determines a coral's response to environmental change. Symbiodinium types can be characterized as a specialist (exclusive to particular host), or generalist (common amongst many hosts). These different partnerships may have important implications for the functioning of the holobiont. We hypothesize that under variable environmental conditions, specific lineages are prone to competition and displacement by generalist types, which have a higher adaptive plasticity and are more resilient to disturbances. Alternatively, specialists may perform better and outcompete generalists when conditions are optimal and stable. Owing to its sub-tropical latitude, Hong Kong has a highly variable seasonal fluctuation in temperature (14-31 °C) and salinity (24-32 psu). To test the hypothesis that environmental variation favours generalization, we identified Symbiodinium from 14 species of common scleractinian corals in the region, representing 12 genera and 7 families by direct sequencing of the ITS2 sub-region of nuclear ribosomal DNA. Our results showed that clade C was the dominant Symbiodinium, with subclade C1 being a generalist type associated with most coral species, whereas C15 and C21 were specific to host coral species Porites lobata and Montipora peltiformis, respectively. Symbiodinium clade D was found only in one species, Oulastrea crispata, which is associated with both subclades C1 and D13. The results indicate a low prevalence of symbiont specificity in Hong Kong hard corals.